To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like.
In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The next generation mobile communications technology has advantages of very simple network architecture, negligible signal processing and round-trip time delay, and extremely high quality of communications and transmission rate etc. According to different downlink and uplink service multiplexing scenarios, general communications system embodies various duplexing modes, namely, Time Division Duplex (TDD), Frequency Division Duplex (FDD) and Hybrid Division Duplex (HDD).
The TDD mode refers to the scenario that the uplink and downlink transmissions occupy the same frequency band, while they are separated in different transmission intervals. A Guard Period (GP) is inserted between the uplink and the downlink transmission intervals.
The FDD mode refers to the scenario that the uplink and downlink transmissions occupy different frequency bands such that simultaneously transmitted uplink and downlink signals are separated by different carrier frequencies. A Guard Band (GB) is inserted between the frequency bands occupied by the uplink and the downlink.
The HDD mode integrates the TDD mode and the FDD mode, in a cell of which carrier frequencies are constructed in pairs, a user terminal communicates with a base station on a master carrier and a slave carrier according to a predetermined pattern. Specifically, if the sub-frames on the slave carrier are all uplink sub-frames, the user terminal communicates with the base station on the master carrier and the slave carrier in FDD mode; if the slave carrier is time multiplexed of downlink and uplink sub-frames, the user terminal communicates with the base station on downlink resources of the master carrier and uplink resources of the slave carrier in FDD mode, and/or, the user terminal communicates with the base station on downlink resources of the slave carrier and uplink resources of the slave carrier in TDD mode.
FIG. 1 is a schematic diagram illustrating a frame structure of TDD mode in the Long Term Evolution (LTE) system corresponding to the Evolved Universal Terrestrial Radio Access (E-UTRA) protocol developed by the 3rd Generation Partnership Project (3GPP).
Referring to FIG. 1, the length of one radio frame is 10 ms, and one radio frame is composed of ten sub-frames including special sub-frames and normal sub-frames, the length of each sub-frame is 1 ms. The special sub-frames are classified into three intervals, namely a Downlink Pilot Time Slot (DwPTS), a GP used between the uplink and downlink, and an Uplink Pilot Time Slot (UpPTS). The normal sub-frames include uplink sub-frames and downlink sub-frames, which are used to respectively transmit uplink and downlink control channels and data channels. In one radio frame, it is possible to configure two special sub-frames (i.e., sub-frame 1 and sub-frame 6), or configure one special sub-frame (i.e., sub-frame 1). The sub-frame 0, sub-frame 5 and DwPTS in the special sub-frame are always used for downlink transmissions, the sub-frame 2 and UpPTS in the special sub-frame are always used for uplink transmissions, and the other sub-frames can be flexibly configured as either downlink sub-frames or uplink sub-frames according to actual needs.
In particular, the LTE TDD system supports seven uplink-downlink configurations, as shown in table 1, “D” indicates a downlink sub-frame, “U” indicates an uplink sub-frame, and “S” indicates a special sub-frame. It can be seen from table 1 that the ratios of uplink sub-frames to downlink sub-frames of the seven uplink-downlink configurations are different. The uplink-downlink configuration 5 has the most downlink sub-frames, and the ratio of downlink sub-frames to uplink sub-frames is 9:1; the uplink-downlink configuration 0 has the most uplink sub-frames, and the ratio of uplink sub-frames to downlink sub-frames is 3:2.
TABLE 1Uplink-Downlink-downlinkuplinkconfigurationtransitionSub-frame indexindexperiod012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
In a conventional TDD system, the allocation of uplink and downlink sub-frames is conducted in a static or semi-static manner. The common approach is to determine the ratio of uplink sub-frames to downlink sub-frames according to deployment scenarios and service requirements during the phase of network planning, and remain unchanged. This approach is relatively simple and effective in the scenario where only macro cells are deployed. However, this approach is no longer effective in future 5th Generation (5G) communications system where small cells are densely deployed (so-called Ultra-dense Network (UDN)). In UDN, fewer users exist in each cell, and the change in traffic demands in each cell is very significant. Therefore, there is a need to adaptively adjust the ratio of downlink sub-frames to uplink sub-frames or vice versa to suit for the traffic variations.
In a multi-cell deployment scenario, if different cells adopt the same uplink-downlink configuration or the uplink-downlink transmission directions of adjacent cells are the same at a given time instant, the user terminal and the base station will respectively suffer from inter-cell interference as shown in FIG. 2a and FIG. 2b on an uplink sub-frame and a downlink sub-frame.
FIG. 2a shows the Type-I inter-cell interference 210, namely, on a sub-frame where the adjacent cell performs the downlink transmission, the downlink reception of the user terminal in the cell of interest suffers from interference caused by the downlink transmission of the base station in the adjacent cell.
FIG. 2b shows the Type-II inter-cell interference 220, namely, on a sub-frame where the uplink transmission is performed in the adjacent cell, the uplink signals, which is transmitted by the user terminal, received by the base station of the cell of interest suffer from interference caused by uplink transmission of the user terminal in the adjacent cell.
In addition, in a multi-cell deployment scenario, if adjacent cells adopt different uplink-downlink configurations or the transmission directions of adjacent cells are different at a given time instant, the base station or the user terminal will suffer from inter-cell interference as shown in FIG. 2c. 
FIG. 2c shows the Type-III inter-cell interference 230, namely, on a sub-frame where the unlink-downlink transmission directions of adjacent cells are different, the uplink signals, which is transmitted by the user terminal, received by the base station of the cell of interest suffer from interference caused by downlink transmission of the base station in the adjacent cell.
FIG. 2c further shows the Type-IV inter-cell interference 240 as well, namely, on a sub-frame where the unlink-downlink transmission directions of adjacent cells are different, the downlink reception of the user terminal in the cell of interest suffers from interference caused by the uplink transmission of the user terminal in the adjacent cell.
The existence of the so-called cross-slot interference (Type-III and Type-IV interference) restricts the flexibility of dynamically configuring the uplink and downlink sub-frames in the TDD system. To solve above mentioned problem, the 3GPP has initiated a working group (WG), named “Enhancements to LTE Time Division Duplex for Downlink-Uplink Interference Management and Traffic Adaptation (eIMTA)” on May 2010, to investigate methods and associated signaling supports that enable traffic adaptation and interference mitigation in dynamic TDD system. The interference management methods studied by the eIMTA WG include cell clustering, frequency domain multiplexing, power control, and so on. These interference management methods are especially used for eliminating or avoiding the Type-III interference between base stations. This is because the eIMTA WG believes that, compared to the Type-IV interference between user terminals, the Type-III interference between base stations has a far greater impact on the system performance. This is mainly because: 1) the transmit power of the base station is generally higher than that of the user terminal, and channels between base stations are usually Line-of-Sight (LoS) channels; 2) according to the statistical theory, the frequency at which the Type-III interference appears is higher than that of the Type-IV interference appears; 3) the Type-III interference is inter-base-station interference, and is easy to manage and control, while the Type-IV interference is inter-user-terminal interference, and is difficult to manage and control.
In order to improve the system throughput, enhance the spectrum efficiency in future 5G communications system, the deployment of UDN has become a trend. In particular, for the 5G communications system, the consistency between cell-center and cell-edge performances is an important performance indicator to evaluate the overall system performance. Hence, if adjacent cells adopt different uplink-downlink configurations, besides the Type-III inter-base-station interference, the Type-IV inter-user-terminal interference also to a great extent degrades the cell-edge performance of the system. This is due to the fact that with increase in the number of cells and reduction in cell coverage, the equivalent distance between user terminals in different cells will be shortened accordingly. That is to say, if adjacent cells employ different uplink-downlink configurations, the frequency at which the Type-IV interference appears and the interference level in 5G systems is considerably higher than the frequency at which the Type-IV interference appears and the interference level in 4G systems, which cannot be ignored.
In conclusion, regarding the inter-user-terminal cross-slot interference mitigation and asymmetric traffic adaptation of cell-edge user terminals, there have been no effective solutions available in the literature.